Knowledge of the dynamic behavior of lipid molecules is of great importance for a proper insight into the functional properties of biological membranes. An understanding of the structure-function relationships in membranes will lead to a better understanding of such processes as nerve transmission and brain function. the dynamical behavior is often studied in model systems consisting of supramolecular assemblies of phospholipid molecules. We intend to study slow motion of phospholipid molecules in spin labeled model membranes using the double modulation of electron spin resonance (DMESR). The DMESR technique is simple to use and could be readily available, because a DMESR spectrometer is a simple extension of a commercially available electron spin resonance (ESR) spectrometer. The DMESR signal is the homogeneous spin packet line whose linewidth depends on the motion of the spin labeled molecule, spin-spin and dipolar interactions, but it does not depend on molecular orientation. Therefore, the DMESR technique is very suitable for the study of slow motion of phospholipids in membranes. The main advantage of DMESR over the other continuous wave ESR techniques in the study of slow motion in membranes is the ability to separate the molecular order effect from the molecular dynamics effect. In particular, DMESR will be applied in the study of the anisotropy of motion in spin labeled phospholipid multibilayer and liposome model membranes, and the segmental motion along the phospholipid chains. We will also use the DMESR technique in the study of the slow motional component in the system consisting of slow and rapidly tumbling spin labeled lipids. A full theoretical understanding of the DMESR signal is still lacking. We propose to perform computer simulations of the DMESR signal using the Bloch equations in the complex form. Especially, we expect that the study of two microwave field ESR, which is the simplest case of DMESR, will give us better understanding of the phenomenon of DMESR. All computer simulation results will be compared with the experimental results. For the first time, a double modulation ESR on a homogeneous line will be performed. The study of technique alone will improve our analysis of the results of DMESR measurements in spin labeled model membranes.